In the glass industry today the most common glass container manufacturing machine is the Hartford type "I.S." machine. The basic I.S. machine is described in U.S. Pat. Nos. 1,843,160; 1,911,119; and 2,289,046.
In a conventional Hartford I.S. type of glass bottle machine, a purality of individual sections are operated in timed relationship to one another. Individual gobs of molten glass from an automatic feeding device are fed to the blank mold side of each of the stations. Each section has one or more upwardly opened blank molds for receiving the molten gob, or groups of gobs. The machine is capable of forming glass bottles from these molten gobs by either the well known blow-and-blow process or the well known press-and-blow process. Generally, the blow-and-blow process is used to make narrow neck bottles and the press-and-blow method is used to make wide mouth bottles or jars. Because the press-and-blow process produces bottles with more uniform wall thickness, it offers an opportunity to reduce the weight of narrow neck bottles, but technical factors have made it difficult and, in many instances, impractical to do this. Examples of this process are disclosed in Mumford, U.S. Pat. No. 3,216,813; Becker U.S. Pat. No. 3,622,305; Becker U.S. Pat. No. 3,644,111; Rowe U.S. Pat. No. 3,765,862 and Foster U.S. Pat. No. 4,009,016.
An early attempt to form a blank using upper air pressure and a lower plunger is disclosed in Loper et al. U.S. Pat. No. 1,197,837. In the Loper et al. method a lower plunger was extended upwardly into the blank mold cavity after the gob of molten glass was already received in the blank mold cavity. Other refinements have been made in order to better control the process and in an effort to make the blown bottles lighter in weight and more uniform in wall thickness. Prior art apparatus which is directed to parison forming is disclosed in Wendle U.S. Pat. No. 3,271,127 and Northup U.S. Pat. No. 4,336,050.
It has been known that under prior art systems the successful operation of a glass container narrow-neck press-and-blow process producing a bottle having a 26 mm crown finish or 28 mm thread finish, which are the prominent beer and beverage sizes, depends on the close maintenance of the volume of the blank mold cavity and of the pressing plunger. Furthermore, the weight of every the gob of glass had to be held to an extremely close-tolerance specification. Most bottle manufacturers do not attempt to operate the press-and-blow process on finishes below 38 mm in size because the cost of maintaining the close specifications exceeds the economic advantage of being able to reduce the weight of the glass in the bottle. For example, maintaining individual gob weight to a close tolerance became more complex as the number of gobs being ejected simultaneously from the feeder was increased. It is very difficult for a manufacturer to attempt to run a triple gob narrow-neck press-and-blow or even to consider a quadruple gob operation.
The need for the close gob specification control, under prior art apparatus and methods, results because the inside diameter of the bottle finishes in beer and beverage sized bottles is only three quarters of an inch, necessitating a pressing plunger diameter only slightly larger. The pressing plunger has to be tapered in order to be withdrawn from the glass after the blank or parison has sufficiently chilled, because the glass shrinks around the plunger as it cools.
In these prior art press-and-blow processes, whenever the volume of a hot glass gob exceeded the volume of the blank and finish cavity minus the volume of the plunger, an over-press occurs, creating a highly objectionable rough seam on the finish of the container. If the volume of the hot glass gob was less than the volume of the blank and finish cavity minus the volume of the plunger, proper pressing did not occur because the fluid mass of glass was not confined. In addition to over and under variations in gob volume, the volume of the plunger in the blank and finish cavity also varied because of differences in the penetration of the plunger.
When attempting to run a triple gob operation or other multiple gob operation, the normal variations in gob volume and plunger penetration when using prior art apparatus and the press-and-flow method often resulted in unsatisfactory results when attempting to produce narrow-neck bottles.
The primary object of the present invention is to produce glass bottle parisons in a narrow-neck press-and-blow operation by providing an apparatus and method that is more tolerant of glass gob weight variations.
Generally in my invention, this objective is accomplished by initially extending a cavity plunger, having a volume of at least 15% of the volume of the blank mold cavity, into the blank mold cavity prior to introducing the gob of molten glass. The plunger is used to produce a cavity in the glass parison and to cool the surface of the interior wall of the glass parison. After introduction of the molten glass into the loading cavity which is located immediately above the blank mold cavity, the gob is forced into the blank mold cavity by air pressure, either positive or negative. This force is applied after the molten glass gob has made contact with the wall of the loading cavity and created a seal. In the preferred embodiment, compressed air is applied to the gob from a blow head and baffle assembly which moves into position at the top of the mold. This fluid force moves the glass from the loading cavity into the blank mold cavity. The air presses the molten glass into contact with the mold cavity walls and into contact with the plunger with a force similar in degree to the pressing force of prior art processes. After pressing and cooling, the air is shut off and the plunger is retracted slightly. Compressed air is then blown into the finish of the parison to expand the parison slightly to compensate for gob weight variation and to force the molten glass firmly against the cavity walls and against the mold surface of the baffle to provide the necessary "chill" and to complete the parison formation. After this, the air is shut off, the baffle retracts, the plunger retracts completely, and the blank mold opens. The completed parison is then removed from the parison forming-area by movement of the finish mold.
Under the present apparatus and method, variations in the gob weight do not have the same impact as was true in most prior art narrow neck press-and-blow processes. The plunger is extended prior to pressing. Therefore there is no difference in plunger volume from one operation to the next. If the volume of the hot glass exceeds the specified volumeover-press does not result. Similarly, if the amount of the hot glass in any particular gob is less than the specified weight, the fluid mass of the glass is still confined and proper pressing still results. Therefore, the present apparatus and method is suitable for a high production, economic, high multiple gob narrow neck press-and-blow operation.